Change pivot point for selected edges
You change the pivot point in edit mode by clicking on the pivot point icon in the information header (OK, in the 3D viewport it's actually a footer) and selecting the option you want. In this case, in object mode you can use set origin (SHFT-CTRL-ALT-C to set the origin where you want it, then set the 3D cursor to the origin using the snap tool (SHFT-S) selecting "cursor to active, because in object mode if vertices in the object are selected, the object is active, and the cursor snaps to the origin. The 3D cursor does not move when you go back to edit mode, so you can use the 3D cursor as the pivot point.

In the illustration you included with your question, I assume the blue dot is the origin point, so the 3D cursor is already where it will be, and all you need do is as suggested in the comment: change the pivot point to the 3D cursor in the "pivot point for rotation scaling" in the 3D viewport header..